Can a user get the Firebase userId of one of his contacts that also has the same app installed?

Hi Koders!
For a certain funtionality in my app, I need that a user can send a file (a dictionary) to another user.

I think I can do this using Firebase; the sender user writes in a specific folder of the receiver user and the receiver retrieves this dictionary when the app is initialized

I want to store the data of every user under his userId, so, it is posssible for an user to get the userId of another user?
The receiver has to be in the contacts list of the sender and the receiver, logically, has to have the app instaled too.

Can this be done? Is this a good approach or there is a better way to do the same?
Thanks in advance!

UserIDs are hard to remember and too lengthy too type, so instead of using UserID for finding a person why not use usernames or mobile numbers.
Suppose A wants to talk to B he will search for either the username or mobile number and then the app has to find out the number/username is associated to which userID, and then rest will be the same as you would do earlier with UserID

Thanks for answer!
Not all accounts are related to a phone number, thats why I prefer userId. But its true that if the user wants to share the dictionary he has to do it through the phone number or username, so this is a good idea.

How this search can be performed?

Thanks again!